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12 6 RE SIDENT~L SIBS ON GROUT 8.0 DESIGN OF TYPE IV SLABS Design procedures for this type of slab need little elaboration, since it is a framed slab in the engineering sense-supported with- out contacting the soil on the site. However, some points of caution bear repeating. This slab is appropriate for use with any soil, since it does not rest on soil anywhere over its entire area. It is designed in accord- ance with conventional engineering practices, i.e., supported on piles, piers, or footings which rest on unyielding stable soil or rock. In areas of highly expansive soils, contact should not be permitted with slab or grade beams; otherwise pressure sufficient to damage the slab may result. In addition, it is advisable to pro- vide protection-by the use of belled reinforced caissons, greased tubes, or other means-to reduce the effect of friction on piers or piles passing through expansive soils. PART B: Quality Control 1.0 GENERAL Satisfactory performance of slabs-on-ground cannot be assured by design alone. Appropriate control of site preparation, and of qual- ity of materials and workmanship at each step in the construction process, is of no less importance. While the loads normally en- countered with a residential slab-on-ground will be relatively light as compared with those in larger multi-storied buildings, it is nevertheless true that carefully designed house slabs can be and are damaged through lack or inadequacy of quality control. Thus, the quality and soundness of the finished slab are dependent to a large extent on procedures often ignored or compromised. Practices contained herein are offered as guides; experience indicates that adherence will result in a finished slab which per- forms as intended by the designer.
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SUPPLEMENTARY INFORMATION 127 2 . O SITE PRE PARATION It is vital that the soil upon which the slab is to be placed be of uni- form density over and through the entire slab site. The method used is secondary to the uniformity and degree of compaction achieved. Failure to obtain uniform compaction will lead to soil settlements and induced stresses in the slab which cannot be structurally ac- commodated. The degree of compaction needed varies with the plasticity index of the soil (PI), and the anticipated climatic varia- tion, since, as has been adequately demonstrated, different soil types react differently to changes In moisture. If proper advantage is to be taken of this characteristic in assuring uniform soil support for the slab, careful attention should be given to preparing the soil to receive the slab. The distortions of a soil mass because of heaving become in- creasingly pronounced with increases in density. On the other hand, distortions due to settlement as a result of loading are likely to occur in a loose soil mass. Of these two effects, heaving can potentially induce effects more destructive to the superstructure. Further, the potential for heaving increases with the PI of clays. In these circumstances, the required degree of density, on a site expected to receive construction loads, should be as high as possi- ble-in order to limit settlement-for soils not susceptible to heav- ing. On the contrary, for a soil mass in relatively looser condition that can better accommodate the potential heaving action within its volume of voids, the degree of compaction of soils subject to heav- ing (because of soil consistency or weather or both) should be lower than for time-stable soils. In order to provide guidance in this respect, an empirical chart has been developed, which, on the basis of engineering judgment and experience, provides for me needed degree of compaction in terms of the climatic rating (Cw) and PI of the soil. Figure 22, p. 128, graphically portrays this empirical relationship. A varia- tion of 2% from the optimum density should be considered the maxi- mum acceptable. However, since soils compacted to 80% of ASTM D698-63T or 75% of D1557-64T (or latest editions thereof) will have a relatively low qu, such materials, when used in a fill, should be subjected to analysis by a qualified soils engineer for both com- pressibility and potential heaving effects. The amount of compac- tion required can best be determined by proper analysis and labo- ratory tests conducted by a qualified soils engineer. A simple test that has proven helpful in determining the proper moisture content for compacting clay soils is to squeeze a sample
OCR for page 128
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SUPPLEMENTARY INFORMATION 129 of the material. The sample should be wet enough to hold together when squeezed, but not wet enough to squeeze through the fingers. This is a rough guide and merely an aid in determining satisfac- tory moisture content for the compacted soil. Highly organic soils should never be used as foundation soils for slabs and must be removed from the slab site, since they can- not be satisfactorily compacted and are extremely compressible because of the decay of organic matter. 2 .1 Fills 1 Clear distinction should be drawn between an uncontrolled fill as a slab site, a controlled fill, and a cut-and-fill. Each has its own peculiarities and needs to be treated accordingly. 2.1.1 Controlled Fills Controlled fills are those placed under specified conditions and under qualified supervision and testing. The qualities of such fills are therefore known, and their properties are generally more uniform. 2.1.2 Uncontrolled Fills Uncontrolled fills are those which do not meet the control re- quirement. They are subject to nonuniformity, and their qualities as foundation support cannot be accurately predicted unless veri- fied beforehand by appropriate soil investigation methods. The controlling type of soil to be used for slab selection and/or design purposes should be determined in such fills by the necessary field and laboratory tests. 2.1.3 Cut-and-Fill A cut-and-fill is one in which the soil is excavated from one area of the slab site and redistributed to another area on the site in order to provide a level surface. When this type of fill is em- ployed, it is necessary that the fill and natural soil provide the same degree of support over the slab site The fill material should be placed as a controlled fill. Where cuts and fills involving plastic soil (PI > 15) are proposed Appendix C, Reference 6, Publications No. 1076 and 1281.
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130 RESIDENTIAL SLABS ON GROUND as support for structures, the services of a qualified soils engineer should be required for analysis, testing, and placement supervision. 2 .1.4 Construction of Fills 1 To ensure uniformity, fills should be constructed in shallow in- crements which are tested adequately in order to eliminate the possibility of loose layers. 2.1.5 Fill Materials Where site conditions require fill materials, such fill should conform to one of the following conditions: a. Clean granular type such as gravel, crushed stone, or sand, uniformly graded with 2 - inch maximum size. b. Any clean fill material (soil free of organic material and rubbish) installed under competent supervision with mechanical equipment. 2.2 Natural Moisture Control Variation in moisture content is an important contributor to soil behavior; therefore, every effort should be made to eliminate situ- ations which contribute to variation. With expansive soils, the wa- ter content required during placement needs to be maintained until foundations (slabs) are placed. Whereas nothing can be done to control the occurrence of the natural elements, much can be done to limit their effect on soil behavior. 2.3 Site Drainage Unless proper drainage is provided for the slab site, free surface water will accumulate around and under the slab. This water will tend to consolidate silts and coarse-grained soils and cause expan- sive soils to increase in volume. Precautions are necessary to provide positive drainage away from the slab perimeter, in order to maintain the best possible state of moisture equilibrium. The bottom of the surface slab should be a minimum of 6 inches above the surrounding outside finished grade. The ground should be sloped 1See footnote 2, p. 129. .N
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SUPPLEMENTARY INFORMATION 131 down and away from the edge of the slab for 25 feet at a 2% slope, or a minimum drop of at least 6 inches. 3.0 SLAB MATERIALS The quality of materials to be used in slab construction is of key importance. While the quality of materials used in concrete is generally taken for granted, attention should be directed to the minimum specifications for materials upon which these recommen- dations are based. The latest revisions of the following American Society for Test- ing and Materials Standards should be used for slab materials: ASTM C-33 - Specifications for Concrete Aggregates ASTM C-330 - Specifications for Lightweight Aggregates for Structural Concrete ASIM C-150 - Specifications for Portland Cement ASTM C- 175 - Specifications for Air- Entraining Portland Cement ASTM C-94 - Specifications for Ready-Mixed Concrete ASTM C-205 - Specifications for Portland Blast-Furnace Cement ASTM C-260 - Specifications for Air-Entraining Admixtures for Concrete ASTM A-15 - Specifications for Billet-Steel Bars for Concrete Reinforcement ASTM A-16 - Specifications for Rail-Steel Bars for Concrete Reinforcement ASTM A-160 - Specifications for Axle-Steel Bars for Concrete Reinforcement ASTM A-185 - Specifications for Welded Steel Wire Fabric for Concrete Reinforcement. 3.1 Concrete Quality Mix proportions used should produce a concrete which is workable and readily finished. Concrete should have low permeability, good wear and abrasion resistance, and sufficient durability to withstand local atmospheric and exposure conditions during construction. It
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132 RESIDENTIAL SLABS ON GROUND is generally possible to obtain these characteristics in a concrete having a compressive strength of 2500 psi when tested in accord- a~ce win ASTM Designation C 39.1 For Type II and IV slabs, the compressive strength, when tested in accordance with ASTM C-39, should be in accordance with the ultimate strength used in the structural design. 3.2 Test for Concrete Consistency The consistency of concrete should be measured in accordance with ASTM Designation C-143.2 3.3 Admixtures Admixtures should be permitted when they contribute materially to one or more of the following in fresh or hardened concrete (and provided the benefits derived do not entail adverse effect on other concrete properties recommended in this report): workability, placeability, ease of finishing, strength, durability, lowered absorp- tion or permeability, increased abrasion resistance. 4.0 CONCRETE PRAC TIC ES Placement practices control to a very great extent the service- ability of any slab. To this end, such practices have been related to levels of technical competence in proportioning and mixing which may be prodded on the job. 4.1 Engineer or Architect Supervision Where proportioning, mixing, and placing of concrete are performed under control of a competent representative of Me architect or 1ASTM Designation C-39, Method for Test for Compressive Strength of Molded Concrete Cylinders. Philadelphia: American Society for Testing and Materials. 2ASTM Designation C-143, Method of Test for Slump of Portland-Cement Concrete. Philadelphia: American Society for Testing and Materials.
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SUPPLEMENTARY INFORMATION 133 engineer, the following ACI Standards should be considered to apply: 1 1. Recommended Practice for Selecting Proportions for Con- crete (ACI 613) 2. Recommended Practice for Selecting Proportions for Struc- tural Lightweight Concrete (ACI 613A) 3. Recommended Practice for Measuring, Mixing, and Placing Concrete (ACI 614) 4. ACI Manual of Concrete Inspection (1957~. 4.2 Ready-Mix Concrete without Engineer or Architect Supervision Where proportioning, mixing, and placing of ready-mix concrete are not under the control of an architect or engineer, the following criteria should be considered to apply: a. Concrete, with or without admixtures, should equal or sur- pass-in strength, durability, impermeability, and ease of finish- ing-plain concrete having a ratio of not more than 6 gallons of water per sack of cement. b. The slump of the concrete as designed and as placed on the job should be not less than 3 nor more than 6 inches; for light- weight concrete, the slump should be 1 to 3 inches when tested in accordance with ASTM Designation C-143. c. The contractor should submit to FHA signed delivery tickets for ready-mix concrete, attesting to compliance with the specifi- cations. d. Under some conditions, such as hot, dry weather with long waiting periods between mixing and placing of concrete, it may be necessary to add water to the mix at the job site to regain rec- ommended slump after water has evaporated from the mix. In such instance (in order not to affect the quality of concrete), only suffi 1Separate publications of the American Concrete Institute, Detroit, Michigan.
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134 RESIDENTIAL SLABS ON GROUND cient water should be added to regain the specified consistency, provided the water-to-cement ratio is not exceeded. 4.3 On-Site Mixing Where concrete is to be mixed on a volumetric basis, and not under the control of a competent representative of the architect or engi- neer, the proportions in Table V, p. 135, should be used as a guide. The contractor should submit signed statements to FHA, attesting to compliance with Table V. The statement should show total gal- lons of water and cubic feet of coarse and fine aggregate used for each sack of cement. 4.4 Concrete Placing and Finishing The concrete should be distributed and placed in such a manner that it will work readily into corners and angles of forms, and around reinforcement, without permitting materials to segregate or allowing excess free water to collect on the surface. After placing, concrete should be thoroughly consolidated by spading or vibration, after which it should be screeded to proper grade. Ex- cessive spading or vibration should be avoided, to eliminate risk of separation of materials. The concrete should be worked with a float to remove high spots and to fill depressions. After floating, various finishes can be applied according to standard practices. Where a smooth, troweled finish is required, the surface should be sufficiency hard not to be marred by the weight of the machine. 4.5 Curing Practices Curing should commence as soon as concrete has set sufficiently to prevent damage to the surface. To prevent too rapid drying, the concrete slab should be covered as soon after placement as is possible without marring the surface. Materials such as moist burlap, canvas, cotton matting, liquid membranes, foaming compounds, polyethylene sheeting, or water- proof curing paper with edges sealed, may be used to cover the con- crete during curing. If burlap, canvas, or cotton material is used, cover material should be kept moist. At no time during the curing period should concrete be exposed directly to the drying actions of sun or wind.
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